In the mining industry, and other industries, it is the practice to drill bore holes into the earths' surface, usually for the purpose of extracting cores to determine the nature and mineral content of the substrate being drilled. Usually this is an ore body. In other cases it may be for purposes of examining the earth to place foundations or the like.
It is well known that when drilling to any significant depth, that the drill has a tendency to wander. As a result, the bore hole is in many cases "off-course" and may define a progressive curve whereas it is desirable that the bore hole should be straight.
The practice in core drilling is to employ a series of lengths of cylindrical drill rod, with the drill bit itself being located at the leading end. The drill bit and the rods are hollow, so that they drill out a core of material which then passes internally along the cylindrical interior. Within the cylindrical interior of the drill rods there is a container known as a core barrel. This core barrel receives the core of material which is drilled out by the drill bit. At periodic intervals for example every six feet or so, the core barrel, containing the core of material, is retrieved from the interior of the cylindrical drill rods, and is logged as to depth and is placed in protective casings for subsequent laboratory examination. The retrieval of the core barrel is achieved by means of a wire line, which has captive means adapted to attach to the upper end of the core barrel, so that the core barrel may be withdrawn by the wire line up through the drill rods. Typically the core barrel will have at its upper end some form of attachment device or abutment, and the wire line will have at its lower end some form of capture device, such as spring-loaded claws or the like. By dropping the wire line and capture device down through the drill rods, the capture device captures the abutment on the upper end of the core barrel. The core barrel may then be withdrawn up through the drill rods. When empty the core barrel may be then be returned down through the drill rods and the wire line capture device can be released, leaving the core barrel in the drill rods to receive another length of core material, as drilling proceeds.
This procedure is described here merely for the sake of completeness. It is a well known practice in the art, and has been carried on in this fashion for many years.
Clearly, it is important to know whether the bore hole has gone in the intended direction or whether it has wandered off course. If it is not straight, but curves away from its intended direction then the cores as they are removed from the drill rods will represent a picture of the sub-strate being drilled which is unpredictable, and may give false information to the scientists studying the sub-strate.
In the past, numerous proposals have been made for providing inclinometers and bore hole logging devices. The intention of these devices was to provide a reading of the inclination of the bore hole, as it progressed into the earth. Examples of such proposals are shown in U.S. Pat. No. 3,845,569 Title: Bore Hole Logging Device, Inventor: H. J. Otte et al, U.S. Pat. No. 4,047,306 title: Bore Hole Probe, Inventor: H. J. Otte et al, and 4,467,5256 title: Inclination Instrument, Inventor: H. J. Otte.
In the first U.S. Patent, the instrument was intended to provide both a compass bearing, and also a reading of the inclination of a location in a bore hole. In this way, the device could simply be passed down the bore hole and readings taken an intervals down the bore hole.
The second U.S. Patent relates to a device for logging a bore-hole, after the drill is removed. It is effective, and is still in use today.
In the third U.S. Patent, the device was intended to be attached to the drill rods, at the top end of the core barrel. While the drill rods rotated, during drilling, the inclinometer would not take a reading. However once the rotation of the drill rods was stopped, when the core barrel was full, the device would then take a reading.
The first device was particularly complex and delicate in design. In addition, it did not totally fulfil the needs of the drilling industry. It is particularly desirable to provide inclination readings as each core is removed from the drill hole. However the device shown in the earlier patent was not intended for this purpose. On the contrary, it was intended to provide readings of the inclination of the bore hole either after drilling had ceased or at intervals during drilling.
The second device was intended for the same purpose as the first, but was a much simpler design, and has achieved a wide degree of acceptance in the field. Both these devices were however not as desirable as one that provided a step-by-step reading as each core was removed.
The device described in the later patent was intended to provide a reading for the inclination of each core just before it was removed. However, it was found in practice that the extreme stresses set up by the rapid rotation of the drill rods and the vibration within the drill rods, and the hostile underground environment all combined to cause relatively rapid failure of various moving parts of the device. Consequently, although in principal the device appeared to satisfy industry objectives and requirements, for a step-by-step reading of each core, it was found that it required major servicing at relatively frequent intervals.
Clearly it is desirable to provide an inclinometer instrument which in the first place does not have to be attached to the drill rods and be subjected to the high speed rotation and vibration occurring during drilling.
Secondly it is desirable to provide an instrument which does not require any additional special operation. In particular it is desirable to provide such an instrument which does not require to be inserted into the bore hole and removed, as a separate operation. It is preferable if the insertion and removal of the instrument can take place as part of the normal sequence of drilling operations.